This so-called "Purex Process" of solvent extraction for uranium is well known in the nuclear fuel art and industry. For example, the process is described in the U.S. Letters Patent No. 3,357,802, issued Dec. 12, 1967, and discussed in extensive detail in Chapter 10, entitled "Fuel Reprocessing," namely pages 457 to 514, of Nuclear Chemical Engineering, by Benedict et al, McGraw-Hill Book Company, 1981. The disclosed contents of the foregoing cited patent and section of the text are accordingly incorporated herein by reference.
Briefly, the Purex process consists of a sequence of chemical steps or operations comprising initially treating the waste of scrap material or spent fuel containing uranium compounds with an aqueous solution of nitric acid (HNO.sub.3), and thereby dissolving the uranium to produce uranyl nitrate (UO.sub.2 (NO.sub.3).sub.2) and other acid soluble components within an aqueous phase. This aqueous phase containing the acid dissolved components including uranyl nitrate, and any acid insoluble components of the waste is passed down through an extraction column while an organic phase of tri-butyl phosphate in an organic diluent of a paraffinic mixture such as kerosene is passed up through the extraction column in counter-current flow with the aqueous phase. The soluble uranium compounds comprising uranyl nitrate of the aqueous phase are extracted therefrom by the organic phase and combined with the tri-butyl phosphate. This separates the uranium and carries it within the organic phase from the extraction column. The aqueous phase, and the organic phase each exit from the extraction column at opposite ends from each other and from their respective entries, the aqueous phase with the acid soluble raffinate contaminants and the organic phase with the separated uranium.
The organic phase effluent from the extraction column carrying the separated uranium compounds is then passed up through a stripping column while water is passed down through the stripping column in counter-current flow with the organic phase. The water releases the uranium from the tri-butyl phosphate of the organic phase whereby it is transferred to and carried within the aqueous phase. The aqueous phase, and the organic phase each exit from the stripping column at opposite ends from each other and from their respective entries, the aqueous phase containing the uranium compounds for recovery separated from contaminants. The organic phase is then recycled back through the extraction column. Typically, the procedure is carried out with a continuous flow of all components through the system comprising the extraction column and stripping column.
The desired product of the Purex solvent extraction process is a high purity aqueous phase effluent from the system containing virtually all the uranium of the initial waste fed into the system. However, some losses of uranium occur in the raffinate effluent by design and represent an economic loss. There is an acknowledged "trade-off" between the uranium product purity obtainable and the level of uranium loss in the raffinate. The extent of this balance of benefits depends substantially upon individual design. To enhance impurity reduction, some system designs include an intermediate scrub-section adjoining or as a section of the extraction column.
A decontamination factor (df) may be defined which measure the process capability to remove impurities; the higher the decontamination factor rating for a given impurity the more capable the process in removal capacity. The factor is calculated from parts impurity per million parts uranium in the feed, divided by the some measure in the product stream. Typically, the conventional Purex process is rated in the order of a gadolinia decontamination factor of 30,000.
The columns are typically agitated by either pulse pumps or reciprocating plates to permit optimal droplet formation and coalesence on each plate. This agitation is most commonly referred to as mixing energy. Excessive mixing energy or flow rates can cause flooding, a condition which precludes flow of one or both liquid operating mode phases in the column. Mixing energy is critical to efficiency of the extraction column and helps establish a characteristic uranium profile.
In the Purex process the bulk of the impurity removal or decontamination of uranium compounds is achieved near the inlet of the extraction column for feeding the acid treated waste material. The most efficient operation of the extraction column is substantially at the level of flooding which produces the maximum removal of impurities such as gadolinia. The term flooding refers to a condition in which the two immiscible phases flow countercurrent past each other with a relative velocity that is sufficient to impede the steady flow of one phase or the other phase. However, this renders the extraction column very sensitive to minor changes in the net balance of uranium among all flow streams entering and leaving the column, and practically difficult to control. Excessive uranium loading can aggravate the possibility of flooding, while deficient net uranium loading greatly reduces impurity removal.
When the extraction column is operating at a steady state, a uranium concentration profile therein can be obtained by sampling either the organic or aqueous phase at several points along the vertical length of the column. The profile depends on the degree of trade off chosen between uranium product purity and level of uranium loss.